A magnetic resonance imaging (MRI) system may include two kinds of MRI radio frequency (RF) coils. The first kind of MRI RF coil is a transmit (Tx) coil. A Tx coil, while operating in Tx mode, transmits high power RF energy into the anatomy of the subject being imaged to excite nuclei spins in the tissue being imaged. The second kind of MRI RF coil is a receive (Rx) coil. An Rx coil, while operating in Rx mode, detects weak signals from nuclei spins of the anatomy being imaged. A conventional MRI system uses a built-in whole body coil (WBC) as a Tx coil. In a conventional MRI system, due to the geometric size of the WBC, the WBC applies RF energy to a much larger region of tissue than is required to image a given region of interest. For example, when a head scan is performed and a WBC is used, not only the head, but also the shoulders and chest also receive a high level of RF energy. This creates a high level specific absorption rate (SAR) issue which limits the clinical utility of MRI systems that use a conventional WBC/Rx coil approach. As a result, a local Tx coil is frequently used to mitigate the SAR problem.
A local Tx coil is designed to apply RF energy into only the anatomy being imaged. There are two conventional approaches to transmitting energy from a power source to a local Tx coil. A first conventional approach is to use a direct connection between the power source to the Tx coil using wires. The Direct connection using wires is energy efficient because the energy loss in the connection wires is trivial. A disadvantage of direct connection using wires is that dedicated wiring is required, which increases the cost and complexity of the coil.
A second conventional approach to transmitting energy from a power source to a local Tx coil is to use inductive coupling. For the inductive coupling approach, a primary coil is used to directly connect to a power source. The primary coil may be a WBC or another large coil. The primary coil is a resonant LC circuit. A second coil is also used. The second coil is another resonant circuit and is inductively coupled to the primary coil. Thus, energy can be transferred from the primary coil to the second coil. The second coil can be used to excite nearby anatomy more efficiently than the WBC because the second coil is smaller and closer to the nearby anatomy than the WBC. Compared to the first approach using a direct connection with wires, the inductive coupling approach may be less efficient than direct wiring but is still more efficient than a conventional WBC. One benefit of the inductive coupling approach is that no special wiring is required. However, conventional inductive coupling approaches require the use of multiple coils. For example, a conventional inductively coupled knee coil uses two layers of RF coils. The first (inner) layer includes a plurality of Rx coil elements which detect signals from the anatomy while operating in Rx mode, and which are decoupled from the transmitting field while operating in Tx mode. The second (outer) layer is a standard birdcage coil which inductively couples to a WBC to create a local amplified transmitting field in Tx mode and which is disabled in Rx mode. However, this conventional inductively coupled dual layer coil has drawbacks. For example, all the individual Rx coil elements in a conventional dual layer coil need associated circuits for decoupling the Rx coil and the local Tx coil while operating in Tx mode. Conventional inductively coupled dual layer coils also require circuits for switching off the Tx coil while operating in Rx mode, which require complex and expensive control circuits. This leads to complex and expensive coils. These multiple decoupling circuits and complex control circuits can also decrease the signal to noise ratio (SNR), thereby reducing image quality. Furthermore, the outer layer, by its proximity to the inner layer, will create additional noise when the inner layer is operating in Rx mode.